Problem description
# Description Activity Start End
1 Communicatieplan Write plan 1-01-06 1-06-06
1 Communicatieplan Present 1-02-06 1-05-06
2 Site As/IS Analyze Sit 1-03-06 1-01-07
2 Site As/IS Write plan 1-02-06 1-06-06
2 Site As/IS Present 1-03-06 1-05-06
3 Projectplan Analyze 1-02-06 1-01-07
3 Projectplan Write plan 1-03-06 1-06-06
3 Projectplan Review 1-02-06 1-05-06
3 Projectplan Present 1-03-06 1-01-07

I am looking for a function that can fill in the empty cells below. So, if
number 1 then the lowest overall start date and the overall last end date.
Can anyone provide
me with a function that does exactly this?

Overall Description Overall start Overall end
1 Communicationplan ? ?
2 Site As/IS ? ?
3 Projectplan ? ?